This isn't something you just pick out a card on a whim and slap it in. It helps if it fits your budget, spec, case, and display without being under or overkill.
if you're gaming at 1080p then a 2gb (nvidia gtx 660 or better) card will do, if you want to game at 1440p or 2160p then you'll need a newer 3 or 4gb card